netdev - Re: [PATCH] tipc: Avoid copying bytes beyond the supplied data

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Date:   Sun, 5 May 2019 16:56:58 -0400
From:   Arvind Sankar <niveditas98@...il.com>
To:     Chris Packham <Chris.Packham@...iedtelesis.co.nz>
Cc:     David Miller <davem@...emloft.net>,
        "jon.maloy@...csson.com" <jon.maloy@...csson.com>,
        "ying.xue@...driver.com" <ying.xue@...driver.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "tipc-discussion@...ts.sourceforge.net" 
        <tipc-discussion@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tipc: Avoid copying bytes beyond the supplied data

On Sun, May 05, 2019 at 08:20:14PM +0000, Chris Packham wrote:
> On 4/05/19 4:45 PM, David Miller wrote:
> > From: Chris Packham <chris.packham@...iedtelesis.co.nz>
> > Date: Thu,  2 May 2019 15:10:04 +1200
> > 
> >> TLV_SET is called with a data pointer and a len parameter that tells us
> >> how many bytes are pointed to by data. When invoking memcpy() we need
> >> to careful to only copy len bytes.
> >>
> >> Previously we would copy TLV_LENGTH(len) bytes which would copy an extra
> >> 4 bytes past the end of the data pointer which newer GCC versions
> >> complain about.
> >>
> >>   In file included from test.c:17:
> >>   In function 'TLV_SET',
> >>       inlined from 'test' at test.c:186:5:
> >>   /usr/include/linux/tipc_config.h:317:3:
> >>   warning: 'memcpy' forming offset [33, 36] is out of the bounds [0, 32]
> >>   of object 'bearer_name' with type 'char[32]' [-Warray-bounds]
> >>       memcpy(TLV_DATA(tlv_ptr), data, tlv_len);
> >>       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >>   test.c: In function 'test':
> >>   test.c::161:10: note:
> >>   'bearer_name' declared here
> >>       char bearer_name[TIPC_MAX_BEARER_NAME];
> >>            ^~~~~~~~~~~
> >>
> >> Signed-off-by: Chris Packham <chris.packham@...iedtelesis.co.nz>
> > 
> > But now the pad bytes at the end are uninitialized.
> > 
> > The whole idea is that the encapsulating TLV object has to be rounded
> > up in size based upon the given 'len' for the data.
> > 
> 
> TLV_LENGTH() does not account for any padding bytes due to the 
> alignment. TLV_SPACE() does but that wasn't used in the code before my 
> change.
> 
> Are you suggesting something like this
> 
> 
> -        if (len && data)
> -               memcpy(TLV_DATA(tlv_ptr), data, tlv_len);
> +        if (len && data) {
> +               memcpy(TLV_DATA(tlv_ptr), data, len);
> +               memset(TLV_DATA(tlv_ptr) + len, 0, TLV_SPACE(len) - 
> TLV_LENGTH(len));
> +        }
> 
> 

For zeroing out the padding, should that be done in TCM_SET in the same
file as well? That one only copies data_len bytes but doesn't zero out
any alignment padding.